1. Field of the Invention
The present invention is in the field of optical fiber communication systems, specifically directed to the problems of signal monitoring in networks of the type referred to as transparent optical networks (all-optical networks), i.e. systems without digital intermediate regeneration. The invention serves the purpose of monitoring, calculating or estimating the signal quality of optical signals in optical fiber communication systems in the form of the bit error probability or related quantities such as, for example, the Q-factor or the bit-to-error ratio.
2. Description of the Prior Art
Modern optical fiber communication networks operate according to the wavelength-division multiplex (WDM) principle (Laude, Wavelength Division Multiplexing, Prentice Hall, 1993). For cost reasons, the individual wavelength-division multiplex channels are no longer subjected to a complete digital regeneration in every network node but generally are only purely optically intensified, for example with the assistance of erbium-doped fiber amplifiers (EDFA). During the transmission, the generally binarily intensity-modulated signal is subject to different distortions such as, for example, chromatic dispersion and non-linearities of the optical fibers as well as to channel crosstalk. Further, the signal has the amplified spontaneous emission (ASE) of the optical amplifiers superimposed thereon.
Conventionally, only the ratio of the signal power of a channel to the spectral power density of the ASE, referred to as the optical signal-to-noise density ratio (OSNR—measurable with optical spectral analyzers) has been utilized as the criterion for the signal quality. Since the signal shape does not enter into the OSNR, an adequately precise statement about, for example, the bit error probability (BEP) thus cannot be made.
Another known method employs an optical receiver, i.e. conversion of the optical into an electrical signal, that is then sampled in the bit middle. As shown in FIG. 1 herein, an amplitude histogram is subsequently formed, the averages and variances of the two levels for “0” and “1” being subsequently determined therefrom either by forming the corresponding moments or by interpolation. The variances are allocated to the influence of Gaussian noise (Hewlett Packard Digital Communications Analyzer HP 83480 A, Users Guide, Hewlett Packard, 1995), which leads to a correct result in the calculation of the BEP only given completely undistorted and crosstalk-free signals.